Optical communication systems require high speed data, implemented as optical signals, to be switched between ports of a switching device to allow a signal routing function. Typically, the optical signals are carried by optical fibers, which connect to the optical switching device. There are currently a number of methods for achieving the required switching operation.
One solution comprises an electromechanical arrangement, where a signal in an optical fiber A is routed to fiber B by mechanically aligning fiber A with fiber B. This arrangement is bulky and mostly suited only to 1×N switch configurations.
An alternative solution is to use a hybrid optical switch in which the optical signals are first converted to electrical signals which are switched in a conventional manner. The resulting outputs of the switch are then converted back to optical signals. This adds complexity and expense to the switching operation.
Optical switches are also known in which a control signal is used to vary the path of an optical signal. For example, waveguide-based switches rely on the change of refractive indices in the waveguides under the influence of an external electric field, current or other signal.
Optical switches using an array of mirrors which can be mechanically tilted are also known. Small micromirrors (for example less than 1 mm) are arranged in a line or array, and the incident light signal is deflected by controlling the tilt angle of each micromirror. Mirror type optical switches include digital micromirror devices which tilt each micromirror by electrostatic force, piezoelectric drive micromirror devices which tilt each micromirror by a fine piezoelectric element and electromagnetic devices which rely upon electromagnetic and electrostatic forces.
In a typical micromirror device, a plurality of micromirrors are arranged in an array of N×M mirrors. Each micromirror can be controlled and is capable of switching between a first reflection state and a second non-reflection state. The optical signal is routed between an input and a selected output by controlling the reflection state of each mirror.
Conventional micromirror arrays use highly reflective coatings, such as gold, to provide reflection for all wavelengths of the optical signals.
WDM (wavelength division multiplex) optical communications systems combine signals by providing different channels on different carrier frequencies. These channels are combined into a WDM signal, which is carried on fiber spans across a network. The combining (multiplexing) of channels into a single WDM signal, or the separation (demultiplexing) of channels from the WDM signal, requires optical components capable of separating or combining channels based on the channel frequencies.
In addition to multiplexing and demultiplexing operations, it is often desirable to tap or add one or more individual selected channels from/to a WDM signal, rather than performing a full demultiplex/multiplex operation, for example to perform add/drop operations or for channel monitoring or processing.
The optical add/drop multiplexer, OADM, is a key component in an all optical network (AON). OADM's are designed to allow selection of either a single channel or band of channels from an optical signal to allow routing at a network node. This selection and removal of channels is known as the drop function. Thereafter, the OADM will provide the functionality to allow different data to be reinserted on the wavelengths of the channel(s) that have been dropped, providing the add function. With the functionality of being able, dynamically, to add and drop channels optically, the network becomes optically agile allowing wavelength reuse and rerouting to reflect altering demands on a very short timescale.
Generically, OADM functionality can be achieved optically by demultiplexing the optical signal into several paths, each either carrying a single or a few channels. The switching function therefore need not be channel selective as the channel separation has already occurred.
It has been recognised that it would be preferable to perform the add/drop function without requiring this full demultiplex operation. For example, microring resonators have been proposed as a compact, scalable and integrable way of providing photonic switching functions due to their wavelength selectivity. As Microring resonators (MRR's) are wavelength selective, no demultiplexing is required for their application. They do however introduce some loss and are not switchable devices.